Under certain scenarios this search space may be referred to as a threat space. Typically at least one of the site, object, or medium, such as atmosphere or water, is non-stationary. In particular, versions of the invention described herein will enhance the detection and tracking of surface-hugging moving objects (SHMOs) within an obscuring (capable of at least one of scattering, absorbing, and distorting) or non-obscuring medium. The detection and tracking of non-surface-hugging objects will also be enhanced by implementing versions of this invention. Examples of surface interfaces include the atmosphere-water interface, atmosphere-terrain interface, atmosphere-vegetation interface, atmosphere-building interface, water-ice interface, and water-ocean bottom interface. Two groups of objects that are of particular concern are:
Flying objects such as helicopters, manned and unmanned planes, and missiles, which can include by way of example sea surface skimming, cruise, scud, intermediate, long range, surface-to-air, air-to-air. In particular surface-hugging (low-flying) flying objects (SHFOs) such as missiles and planes moving through a non-obscuring or an obscuring medium while in close proximity to a surface. The surface can include environments such as the surface of the water, terrain, tree tops, buildings, lake, river, ocean bottoms, submerged terrain, biological materials, man-made structures, etc. Examples of an obscuring or non-obscuring medium include the atmosphere, which could alter electromagnetic (EM) signal propagation through scattering, absorption, and distortion. Distortions include refractive turbulence effects due to a non-uniform index of refraction distribution.
Example swimming objects and in particular surface-hugging swimming objects (SHSOs) include fish and marine mammals, frogmen, mines, submarines, remote vehicles, and torpedoes moving underwater in close proximity to a surface. Since water is a type of acoustically obscuring medium, detection of SHSOs can be difficult.
Additional types of objects that can also be detected and tracked using versions of this invention include boats, surface vehicles (bikes, cars, trucks, etc.), and animals (birds, people, etc.).
The present invention can be understood by considering the problem of trying to detect and track a stationary or moving object within a search or threat space. This operation is made more difficult when the object is within an obscuring medium and/or the object is a moving object. An added complication arises when the object is near a boundary or interface between two mediums. A challenging problem involves detecting and tracking surface-hugging moving object (SHMO) within an obscuring medium (a medium such as water or air which is capable of scattering, absorbing, and/or distorting the propagation of acoustic or electromagnetic (EM) radiation, respectively). SHMOs of interest are surface-hugging swimming object (SHSOs) and flying objects (SHFOs). Examples of SHSO include submarines, autonomous underwater vehicles, frogmen, and marine mammals or fish moving through the water (an obscuring medium) while in close proximity to the water-atmosphere or water-ocean/lake/river bottom boundary (including submerged terrain, biological structures, and man-made structures). Examples of SHFOs include missiles or small planes moving through the atmosphere (an obscuring medium) while in close proximity to a surface.
Consider the problem of detecting and tracking SHFOs. Active detection and tracking methods (such as radar) and passive detection and tracking methods, such as detecting infrared emissions or scattered EM radiation due to natural illumination, have been employed with limited success. Missiles and planes are often coated with radar-absorbing materials and/or have shapes which present a low radar cross section. The EM signature of a missile flying close to a surface such as water often contains clutter and speckle or scintillations (when coherent interference conditions are present) due to multipath and atmospheric refractive turbulence effects or distortion. These effects are also present in the passive case since atmospheric refractive turbulence near the surface can distort the propagation path of EM radiation, for example, uv, visible (optical), infrared, and microwave, due to a non-uniform index of refractive and atmospheric scattering and absorption are also present. The cumulative effects from EM scattering, absorption, or distortion (or any combination thereof) can cause the atmosphere to assume the role of an obscuring medium for the purpose of remote detection of a SHFO. A time-varying surface (boundary or interface), as in the case of a body of water, adds another layer of complexity to the multipath aspect of the detection and tracking (or image reconstruction) problem. In the field of optical and infrared astronomy, the local atmosphere distorts the propagating wave front from stars in addition to scattering and absorbing the wave front. Related imaging problems involving the effect of scatter, absorption, distortion, and sometimes multipath interaction with the propagating wave front are encountered in medical and industrial Ultrasound and Ultrasound CT. See Modern Acoustical Imaging (H. Lee & G. Wade, eds. 1986; A. Kak, et al., Principles of Computerized Tomographic Imaging (1988)).
A particular example involves detecting and then tracking a sea surface skimming missile targeting a ship at sea. A sea surface skimming missile is likely to move within a fairly specific search space (threat space) as it approaches a ship. This problem has led to the development of radar systems (radar interferometers) with high angular resolution for the purpose of tracking a sea surface skimming missile. The high angular resolution capability is only of practical value because the viable search space is relatively small. Consequentially, approximate positional and directional information is already available. Unfortunately atmospheric effects and multipath interactions impede detection and tracking with this type of radar. If a passive detection method such as infrared detection and tracking is employed, the earliest opportunity to initiate a defensive strategy is when the missile clears the horizon. Unfortunately, the passive signal from the missile may pass through a substantial distance of a scattering, absorbing, and distorting atmosphere before it is detected. It would be desirable to compensate, either electronically or mechanically, for the contribution to the atmospheric image transfer function (ITF) that is due to refractive turbulence and so improve detectivity. It would also be desirable to compensate for the scattering and absorption aspects of the ITF. For purposes of tracking a SHFO, the atmospheric ITF can be updated as a function of position in the vicinity of the SHFO. The ITF need not be limited to incorporating only atmospheric effects. It can be extended to include multipath propagation due to any nearby surfaces. Active detection and tracking capabilities can be improved by encoding the search space and by increasing the distribution of data samples with respect to viewing angle. Multiple viewing angle imaging techniques such as tomosynthesis, described in computed tomography (CT), Positron Emission Tomography (PET), and Singe Photon Emission Computed Tomography (SPECT) (see E. Christensen, et al., An Introduction to the Physics of Diagnostic Radiology, (1978); J. Liu, et al., IEEE Trans Medical Imaging, Vol. 8, No. 2, pp. 168-172 (1989)) are widely used in medical and industrial radiology.